You may want to rename or otherwise back up the original minigun_fire.wav.
Alternatively to restore the original file, you may simply verify the game's integrity by right-clicking on Left 4 Dead in Steam, clicking Properties, then clicking Local Files and Verify integrity of game cache.
I simply wanted a more realistic minigun sound. The original just didn't sound right to me, so I found what I needed and mixed the samples.
This sounds great and fits the game well. It just lacks the OOMPH of the default and other renditions of Minigun sounds, but that's not exactly realistic like this is. Not that it matters. Great sound!
First I was looking for sounds I could source myself that I could edit to sound right, but I quickly ran into problems. My only good mic seems to be bad now and I don't own explosives. I realized I just wanted a good minigun sound fast. I found a good sample at Soundsnap.com (http://www.soundsnap.com/node/81180) then took the minigun spin-up sound from the game and used it for a background mix. I then set both up to loop well and replaced the original file. Voila! Real minigun.